A truck ploughed into a crowd on a shopping street and crashed into a department store in central Stockholm, killing four people and wounding 15 in what the prime minister said appeared to be a terrorist attack.

A destroyed truck is pulled away by a service car after it was driven into a department store in Stockholm, Sweden. The hijacked beer truck plowed into pedestrians at the central Stockholm department store on Friday, sending screaming shoppers fleeing in panic in what Sweden's prime minister called a terrorist attack.(AP Photo)

Swedish police has placed the man confirmed to be detained after the Stockholm truck attack on Friday under prolonged arrest, it said on its webpage early on Saturday.

The police also said cordons around the crime scene in central Stockholm would remain in place until all investigative measures had been completed.

Prime Minister Stefan Lofven, who cut short a visit in southwestern Sweden to return to Stockholm, said he had strengthened the country’s border controls.

“Terrorists want us to be afraid, want us to change our behaviour, want us to not live our lives normally, but that is what we’re going to do. So terrorists can never defeat Sweden, never,” he said.

The area of the attack in central Stockholm was evacuated, including the main rail station, and remained cordoned off late on Friday. All subway traffic was halted on police orders and government offices were closed.

A Reuters witness at the scene saw police officers put what appeared to be two bodies into body bags.

Bloody tyre tracks showed the path of the truck, which was stolen by a masked hijacker while making a beer delivery to a tapas bar further up Drottninggatan, according to Spendrups Brewery spokesman Marten Lyth.

“We were standing by the traffic lights at Drottninggatan and then we heard some screaming and saw a truck coming,” a witness who declined to be named told Reuters.

“Then it drove into a pillar at (department store) Ahlens City, where the hood started burning. When it stopped we saw a man lying under the tyre. It was terrible to see,” said the man, who saw the incident from his car.

Police said four people had died and 15 were injured. National news agency TT said those hurt included the delivery driver, who had tried to stop the hijack.

Several attacks in which trucks or cars have driven into crowds have taken place in Europe in the past year. Al Qaeda in 2010 urged its followers to use trucks as a weapon.

Islamic State claimed responsibility for an attack in Nice, France, last July, when a truck killed 86 people celebrating Bastille Day, and one in Berlin in December, when a truck smashed through a Christmas market, killing 12 people.

“Hijacking a truck, that has happened before,” Magnus Ranstorp, head of terrorism research at the Swedish Defence University, told Reuters.

“And this is a pretty cunning modus operandi. To drive to Ahlens and stop ... There is a way down to the subway just a few metres away from there, and then you ... can jump on any train you want and quickly disappear.”

Swedish authorities raised the national security threat level to four on a scale of five in October 2010 but lowered the level to three, indicating a “raised threat”, in March 2016.

Police in Norway’s largest cities and at Oslo airport will carry weapons until further notice following the attack. Denmark has been on high alert since the February 2015 shootings. Traffic was restricted on the Oresund Bridge linking Denmark and Sweden at the request of Swedish police.

Neutral Sweden has not fought a war in more than 200 years, but its military has taken part in U.N. peacekeeping missions in a number of conflict zones in recent years, including Iraq, Mali and Afghanistan.

The Sapo security police said in its annual report it was impossible to say how big a risk there was that Sweden would be targeted like other European cities, but that, if so “it is most likely that it would be undertaken by a lone attacker”.